1. Field of the Invention
The present invention relates to a fixing device for use in an electrophotographic apparatus, copier, facsimile apparatus or similar image forming apparatus, and a heat roller for the fixing device. More particularly, the present invention is concerned with a fixing device exhibiting a uniform temperature distribution, saving power, resistive to deflection and collapse, and safely operable, and a method of producing a heat roller for the same.
2. Discussion of the Background
An image forming apparatus of the kind described has a fixing device in which a press roller is held in pressing contact with a heat roller. While the heat roller heated is in rotation, the roller nips a sheet carrying toner thereon between it and the press roller. As a result, the toner is melted and fixed on the sheet. The current trend in the imaging art is toward an image forming apparatus with a power saving configuration. To save power, it is desirable that a heater accommodated in the heat roller be turned on only when the fixing device is used. In addition, the heat roller should be rapidly heated in order to reduce a period of time necessary for the roller to reach a preselected heating temperature, i.e., temperature elevation time.
One of traditional heat rollers accommodates a heat source in the form of a halogen lamp therein so as to be heated indirectly thereby. This kind of heat roller is formed of, e.g., aluminum and has a wall thickness of 1.5 mm to 3.0 mm, so that the surface temperature of the roller may not noticeably vary even during continuous operation. A problem with such an indirect heating type of heat roller is that the heat source cannot beat the roller to the fixing temperature in a short period of time. Another problem is that the heat roller cannot be held at the fixing temperature without increasing power consumption.
On the other hand, heat rollers to be directly heated by their heat sources are taught in, e.g., Japanese Patent Laid-Open Publication Nos. 7-140824 and 6-332333. However, even the direct heating type of heat roller is disadvantageous in that only a limited range of materials are applicable to the roller, in that there is a fear of fire ascribable to unusual temperature elevation, in that the roller is apt to bend with respect to the axis thereof or to collapse in the radial direction, and in that the roller is not durable.
The fixing device should preferably be operable with a commercial power source (100 V or 200 V) from the cost standpoint. With a conventional heating body implemented by a resistance body, it is possible to set a necessary amount of heat by adjusting the source voltage via a transformer. However, several hundred watts of power necessary for the fixing device of electrophotography is not achievable without increasing the size, weight and cost of the apparatus.
Two different approaches are available for solving the above problem. First, the resistance may be adjusted on the basis of the thickness of the resistance body. The kind of approach, however, needs high precision as to film thickness. Silk printing and other various application methods are substitutes for the above approach. Second, the resistance may be adjusted on the basis of the length of the resistance body, i.e., by using a particular wiring pattern. The problem with this approach is that because heat is generated along the wiring pattern, it is difficult to cause the heating body to heat uniformly over its entire surface. Moreover, when the heating body and the base of the heat roller are formed of ceramics or sintered body whose thermal conductively is low, the amount of heat is irregular unless irregularities in the thickness and resistance of the heating body or layer are reduced.
Assume that the heat roller is constituted by a pipe of ceramics, glass or similar material having low thermal conductivity. Then, even if a heating body capable of heating evenly is obtained, it is impracticable to correct rapidly a temperature difference between a portion of the heat roller where a sheet carrying toner thereon passes and a portion where it does not pass. As a result, the temperature distribution on the surface of the heat roller is conspicuous. Such an occurrence can be improved to a noticeable degree if a heating layer is formed in a metallic base having high thermal conductivity with the intermediary of an insulating layer.
A heat roller capable of setting up a uniform temperature distribution in its axial direction is disclosed in Japanese Patent Laid-Open Publication No. 5-278141 and implemented as a heating pipe. The heating pipe is a laminate consisting of a conductive layer in the form of a carbon fiber blade, an electrical insulating layer in the form of, e.g., unwoven cloth, and a layer of polyether imide fibers. To produce the heating pipe, the carbon fiber blade is arranged on a mandrel (cylindrical rod). Then, the unwoven cloth is wound round the blade. Further, the polyether imide fiber layer is interposed between the blade and the unwoven cloth. Electrodes are formed between the blade and the mandrel. Subsequently, the entire assembly is inserted into a pipe formed of, e.g., aluminum and then heated in order to connect the laminate and aluminum pipe. Finally, the mandrel is pulled out of the aluminum pipe.
In the heating pipe, the conductive layer consisting of the carbon fibers and insulating layer are formed on the inner periphery of the aluminum or similar metallic pipe and then affixed to the pipe by a thermoplastic polymer. Laid-Open Publication No. 5-278141 does not teach how the resistance of the carbon fibers is adjusted at all. It is therefore extremely difficult to set the resistance of the carbon fibers when it comes to an actual heating body. If the resistance of the carbon fibers cannot be adjusted for setting up desired power to be consumed by the heating body, then the source voltage for applying a voltage to the heating body must be varied. This cannot be done without resorting to an expensive power source device.
Another problem with the heating pipe is that the heating body is positioned inside of the metallic pipe, increasing the temperature elevation time. To reduce the temperature elevation time, the metallic pipe must be provided with a wall thickness as small as, e.g., 0.4 mm or below. However, the amount of heat generated by the heating body is irregular so far as the disclosure indicates. The irregular heat directly translates into irregular surface temperature on the heat roller, obstructing sufficient fixation of toner on a sheet.
Further, Laid-Open Publication No. 5-278141 does not teach whether or not the heating pipe can implement a heat roller capable of setting up a uniform temperature distribution. Presumably, therefore, it is difficult to use the heating pipe as a heat roller. In addition, it appears that an expensive power source device is indispensable in order to adjust the voltage to be applied to the heating body, as stated earlier.
Moreover, the heating layer is not formed on the outer periphery of the metallic pipe, but formed on the inner periphery of the same. Assume that PFA (perfluoroalcoxy resin), PTFE (polytetrafluoroethylene resin) or similar fluorine-contained resin is applied or otherwise provided on the surface of the pipe in order to form a parting layer, and then heated at 350.degree. C. or above. Then, the heat-resistance thermoplastic resin bonding the laminate (heating body) affixed in the pipe melts and comes off the pipe.